1. Field of the Invention
The present invention relates generally to serging or overedge sewing machines, and more particularly to an apparatus for serging a trouser-fly piece along an arcuate line of stitching and a contiguous substantially straight line of stitching extending along one of its longitudinal edges.
2. Prior Art
A known serging apparatus is schematically illustrated in FIGS. 6-8 inclusive of the accompanying drawings and generally designated at 100, which apparatus essentially comprises a serging unit 101 including a sewing needle 102 for forming an overedge or serge stitching S (FIGS. 7 and 8) and a feed dog 103 disposed in a table (not shown) beneath the needle 102 and cooperative with a presser foot (not shown) to advance a trouser-fly piece F through the serging unit 101, and a trimming cutter 104 disposed immediately upstream of the serging unit 101 for trimming a corner of a leading end Fa of the trouser-fly piece F. The apparatus 100 further includes an elongated guide member 105 having a guide surface 106 extending at an angle relative to a straight line L defining the path of movement of the trouser-fly piece F and extending in alignment with the needle 102 and adapted to guide the trouser-fly piece F into the serging unit 101. Designated at 107 is a fly-piece end sensor disposed diagonally to the upstream left of the sewing needle 102 for sensing the leading end Fa of the trouser-fly piece F being advanced into the serging unit 101. Upon sensing of the leading end Fa, the end sensor 107 sends an electrical signal to a controller (not shown) to start operation of the serging unit 101 not immediately but with a predetermined time lag corresponding to the length of time required for the leading end Fa of the trouser-fly piece F to arrive at the sewing needle 102 from the position of the end sensor 107. The end sensor 107 is adapted also to sense a trailing end Fa of the trouser-fly piece F and send a signal to the controller to discontinue operation of the serging unit 101 not immediately but with a certain time lag for the aforesaid reasons. Designated at 108 is a presser member generally in the form of a ball vertically movable toward and away from the trouser-fly piece F on the table by means of for example an electromagnet. Designated at 109 is a fly piece side edge sensor disposed diagonally to the upstream right of the sewing needle 102 for sensing a longitudinal side edge Fc of the trouser-fly piece F opposite to and remote from the guide member 105 and operatively associated with the controller of the serging unit 101 in such a manner that when the edge sensor 109 senses the presence of the trouser-fly piece F, the presser member 108 is held lifted in a standby position away from the trouser-fly piece F, and when the edge sensor 109 senses the absence of the trouser-fly piece F (when the trouser-fly piece F shifts off to the left of the edge sensor 109 for some reason during the serging operation), the edge sensor 109 sends a signal to the controller to lower the presser member 108 onto and press the trouser-fly piece F, whereupon the trouser-fly piece F is caused to turn about the presser member 108 counterclockwise under the influence of rotational moment produced upon advancement of the trouser-fly piece F by coaction of the feed dog 103 and the presser foot. With this rotational movement of the trouser-fly piece F, the trimming cutter 104 severs the trouser-fly piece F substantially arcuately along its leading end portion, followed by the serging of that portion as shown in FIG. 7. Thereafter, the trouser-fly piece F is oriented to move substantially in parallel with the straight path L of movement and is serged along its straight longitudinal edge until the end sensor 107 senses the trailing end Fb of the trouser-fly piece F and sends a signal to discontinue operation of the serging unit 101.
The foregoing prior art apparatus has a drawback in that since the serging unit 101 is arranged to start operation with a certain time lag upon issuance of the signal from the fly piece end sensor 107, the trouser-fly piece F can be serged from its leading end Fa (as indicated at the solid-line position in FIG. 6) if it is fed fast enough, but if it is fed too slow, the trouser-fly piece F is apt to receive the serge stitching from an end portion departing from the leading end Fa as illustrated by phantom-line in FIG. 6, with the results that there are produced trouser-fly pieces F having irregularly or otherwise defectively finished arcuate serges.
The prior art apparatus has a further drawback in that if the trouser-fly piece F is swerved or displaced from the position of the edge sensor 109 during its advancement along the straight path L after the arcuate or curved serging has been completed, the presser member 108 comes into pressure engagement with the trouser-fly piece F and causes the latter to turn counterclockwise, resulting in a trouser-fly piece F being serged objectionably arcuately along its longitudinal straight side edge Fc as indicated at Fe in FIG. 8.